A handheld projector (sometimes referred to as a pocket projector or mobile projector or pico projector) is an emerging technology that applies the use of a projector module in a handheld device, such as a mobile or cell phone, personal digital assistant, global positioning system (GPS) device, headset and/or digital camera, which has sufficient storage capacity to handle sufficient data, i.e., presentation materials, but little space to accommodate an attached large display screen. Handheld projectors can project digital images onto any nearby viewing surface, such as a wall, and necessarily include one or more light sources, which can be one or more lasers or LEDs. An important design characteristic of a handheld projector is the ability to project a clear and bright image, regardless of the physical characteristics of the viewing surface. In use, handheld projectors can be used to project images such as presentations, photographs, videos, maps, games, etc.
In order to have sufficient brightness, resolution and color quality, the light sources employed in projector modules have to be of relatively high power, on the order of about 0.1 watts or more, when lasers are used as the light source, and 1 watt or more when LEDs are used as the light source. With such high power light sources, a significant amount of heat is generated and thermal management is an important consideration in avoiding degradation in the performance of the handheld device in which the projector module is positioned, as well as discomfort to the user.
In providing for thermal management for a handheld projector, some important considerations relate to the ability to channel sufficient heat away from the projector module itself, in order to avoid overheating of the projector module with resultant loss in function or desired lifespan, while not focusing the heat on a specific area or component of the handheld device. The combination of a heat collector, to draw heat from the projector module, and a relatively high surface area, directional heat spreader, such as one formed of anisotropic graphite, such as compressed particles of exfoliated graphite or pyrolytic graphite, has been found uniquely advantageous.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to commercially as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.